This invention relates to a method for regulating DC current. Specifically, it relates to a telecommunication device for regulating the DC line current on a telephone line to conform to desired parameters.
Telephone systems in countries throughout the world have unique system requirements that need to be met in order to legally sell and use telecommunication devices within their respective borders. One of the commonly known system requirements mandates that when a telephone line goes off-hook (i.e., when the telephone line is in use), the DC current level on the line must reach a certain level within a specified period of time and maintain that level until the call is completed. The DC current level on the line must stay at a certain level in order to be interpreted by the telephone system as an active line throughout the duration of the telephone call. The current rise time and maximum current level are also regulated to prevent damage to telecommunication equipment.
In order to hold a telephone line in the off-hook condition, a specified level of current must be drawn which relates to the voltage level on the line and conforms to a country""s telecommunication requirements. The desired operating current is generally expressed on a graph of current-versus-voltage, known in the art as a load-line. The load-line represents a level of resistance for voltages on a current-versus-voltage graph, allowing a level of current to be determined for a given voltage. FIG. 4 is an example of a current-versus-voltage load-line requirement to keep a telephone line in an off-hook condition. The slope of the load-line on a current-versus-voltage graph is the inverse of the line resistance.
Telephone systems develop a voltage which is a potential impressed on the telephone line between two terminals, commonly known as the tip and ring voltage. As seen in FIG. 4, the desired level of current to keep a telephone line in the off-hook condition can be achieved for a given voltage by setting an appropriate line resistance. The template illustrated in FIG. 4 is representative of the parameters set forth by a country and varies from country to country. The parameters can even change within a country due to changes in a country""s requirements (e.g., if a country updates their telecommunication system).
To conform to established requirements, consumer telephone equipment, such as computer modems and telephones, must be capable of setting the DC line current on a telephone line. One method that has been used to set the DC line current on a telephone line when the telephone line goes off-hook is to place an inductor in series with a resistor across a telephone line connection and then couple the voice circuits to the line through a capacitor. As shown in FIG. 5, a commonly known prior art circuit for setting DC line current comprises resistance RDC, capacitance C and inductance L. Inductor L is chosen to have an impedance over the 200 Hz to 4 kHz voice-band that is much larger than the impedance of the phone line and the capacitor-voice circuit combination. Virtually all the AC current flows through the capacitor and voice circuits. At DC, the capacitor looks like an open circuit and the inductor looks like a short circuit, so RDC sets the DC current level. The circuit of FIG. 5 is less than optimal because of the inherently bulky nature and high cost of the inductor L, the amount of time for inductor L to charge, and the need to change circuit elements in countries with different off-hook current level requirements.
Another prior art approach that has been used to control the DC line current in a telephone system replaces the inductor L of FIG. 5 with additional system components that are smaller and less expensive. The arrangement of components as shown in FIG. 6 can be used to control DC line current and is commonly known in the industry as a gyrator. The prior art gyrator depicted in FIG. 6 can be used to control DC line current without the use of an inductor L. The circuit in FIG. 6 functions like a large inductor across the telephone line and can be used in place of the prior art circuit shown in FIG. 5. The gyrator is implemented with many discrete components such as transistors, resistors, capacitors, and digitally controlled switches located close to the tip and ring telephone line interface. As shown in FIG. 6, the gyrator contains digitally controlled switches DCSC and DCSR used to switch different levels of capacitance and resistance into the gyrator circuit, respectively. By switching different levels of capacitance and resistance into the circuit, the time constant of the circuit can be changed, such that the transistors can be manipulated to provide the correct level of current on the telephone line within a specified period of time. The circuit allows different start up transient times and DC current levels to be adjusted in accordance with a user""s specifications using a single circuit. The DCSC switches affect initial transient settling time and the DCSR switches affect the DC load-line. The adjustability of the circuit is established when the circuit components are installed at the time of manufacture. If the specifications change after manufacture, in order to change the device, components need to be physically changed within the device or an entirely new device needs to be installed.
Recently, a gyrator has been developed using digital processing technologies. By incorporating a gyrator into a digital device, the desired line current parameters can be achieved by adjusting parameters on a country by country basis in software. An example of a digital gyrator is disclosed and described fully in co-pending U.S. patent application No. 09/310,021 filed on May 11, 1999, entitled xe2x80x9cDigital Gyrator,xe2x80x9d having at least one common inventor and assigned to the same assignee as the present application (attorney docket Fischer 16-28-9), and is incorporated herein by reference.
A block diagram of the prior art gyrator is depicted in FIG. 7. The gyrator depicted in FIG. 7 is used to control the DC line current flowing between tip 80 and ring 81 at the interface between the data access arrangement (DAA) 74 and the telephone company central office 72. The system controls the DC line current by first using the DAA 74 to generate an analog signal which represents the DC voltage between tip 80 and ring 81. The analog signal is then converted to digital by the analog-to-digital (A/D) converter 82 located in the coder/decoder (CODEC) 76. The resultant digital signal is then processes by the processor 78 which filters 86 and scales 88 the digital signal to achieve a digital DC current control signal, and combines the digital signal with a computer modem transmit (TX) signal 92. The combined digital signal is then converted back to analog by the digital-to-analog (D/A) converter 84. The resultant combined analog signal is then used to control a current source 94 which places a desired DC line current and an AC modem current onto the tip 80 and ring 81 interface between the DAA 74 and the telephone company central office 72.
Although the digital gyrator 70 depicted in FIG. 7 is capable of setting the DC line current on a telephone line in accordance with the specifications of various countries, a system error can occur in the resulting DC line current seen by the central office 72 between tip 80 and ring 81 that could be potentially problematic. The system error is inherent to the prior art digital gyrator 70 because, in order to control the DC line current with processor 78, a DC feedback path between the DAA 74 and the processor 78 is shared with an AC feedback path. Separate A/D converters could be used for converting the DC path and the AC path, however, it is more feasible to use a single A/D converter 82. Conventional A/D converters 82 often can accommodate only a small range of voltage at their inputs. In addition, present modem specifications (i.e., V.90) require that a modem signal-to-noise ratio (SNR) for the AC path be maintained at greater than 80dB. In order to maintain a high SNR in a gyrator 70 with an A/D converter 82 having a small input voltage range, a majority of the input range of the A/D converter 82 must be reserved for the AC voltage component of the feedback path. This means the DC portion of the feedback path must be as small as possible. In order to decrease the DC portion, the system feedback path may divide the DC voltage between tip 80 and ring 81 by a large number, e.g., 400. However, with DC values this small any DC offset in the analog circuitry, specifically in A/D converter 82, will be interpreted by processor 78 as either reduced or increased tip 80 and ring 81 voltage. Thus, significant error in the resulting DC line current level between tip 80 and ring 81 may result.
The present invention provides a digital method and apparatus for controlling the DC line current on a telephone line with a digital gyrator which has superior error handling capabilities. The invention provides superior error handling by determining the DC offset of a device, storing the DC offset, and subtracting the DC offset out of appropriate calculations performed by a processor. The digital gyrator controls the DC line current parameters with a processor, instead of electrical components such as resistors and capacitors.